Track Matte on stroke skipping frames

Hi, 

I haven't posted a ton on here before, so sorry if this is in the wrong spot. I'm relatively new to after effects, and mostly self taught in this program. I learned how to use track matte with a stroke to reveal an object underneath, and I'm now running into a weird issue. 

I want to make an object appear along a stroke, and then dissapear along the same stroke. I used end keyframes to make it appear, and start to make it disspear, since I needed the mask to start at the beginning. 

At the very end of the "start" transformation, the smooth fade from 0% to 100% seems to skip. When I zoom in, I find that in one frame, it goes from 95% to 100%, without a smooth transition.

I'm duplicating this effect across multiple objects, and it happens every time, but with different jumps: 98%, 87%, 96%, it varies. I even deleted the mask and redid the whole effect, and it's still jumping at the last frame.

This is creating a problem because my object can't smoothly dissapear anymore. I tried extending the time of transition so it moves slower, but the only thing that that does is make the jump smaller, like 98% to 100%. Plus I need this at a fast speed anyway so that doesn't help. 

To me this seems like a glitch, but if it's user error please feel free to tell me what I'm doing wrong here!